In recent years, there have been remarkable developments in the electronics field. In particular, in electronic apparatuses, reduction in size, reduction in weight, increase in density, and increase in output have been advanced. As a result, the demands for these properties and performance have been increasingly growing. It has been desired to increase the high electrical-insulation reliability of electronic circuits and reduce electronic circuits in size in order to increase the density thereof. Further, it has been strongly desired to improve the heat radiation property of electronic apparatuses in order to prevent the degradation thereof caused by increased heat generated in the electronic apparatuses resulting from the increase in output thereof.
In the electronics field, polymeric materials are suitably used as electrically-insulating materials. Therefore, it has been desired to improve the thermal conductivity of such polymeric materials in order to improve the heat radiation property of electronic apparatuses. Although there is a limit to how much the thermal conductivity (heat radiation property) of polymeric materials can be improved, the thermal conductivity (heat radiation property) can be improved further by mixing thermally conductive particles in the polymeric materials. Such materials can be used for adhesive sheets having a thermal conductivity or thermally conductive members and the like such as pressure-sensitive adhesive sheets and the like.
For example, Patent Literature 1 discloses a resin for molding that contains a nano-composite polyamide resin in which a layered silicate is uniformly dispersed, and a thermally conductive inorganic filler. Examples of the thermally conductive inorganic filler include alumina, magnesium oxides, silica, zinc oxides, boron nitrides, silicon carbides, and silicon nitrides.
It has been desired to improve the thermal conductivity of a thermally conductive inorganic filler so that a thermal conductivity is given to a molded object by using a smaller amount of the thermally conductive inorganic filler than that has been conventionally used.
Patent Literature 2 discloses a method for obtaining spherical composite particles having an average particle diameter of 3 to 85 μm and having an improved thermal conductivity by granulating and sintering highly thermally conductive particles having an average particle diameter of 10 μm or smaller.
Specifically, Patent Literature 2 proposes a method for obtaining spherical composite particles in which thermally conductive particles such as alumina, an aluminum nitride, or crystalline silica are coated with a silane coupling agent or a thermosetting resin, and then the coated thermally conductive particles are sintered at a temperature close to the melting point of the thermally conductive particles, i.e., a temperature equal to or higher than 800° C. and preferably at 1000 to 2,800° C. (see paragraphs [0009], [0021]-[0022] and [0028]-[0032]).
It is mentioned in Patent Literature 2 that the composite particles are sintered in order to increase the cohesive force of the composite particles. However, since the thermally conductive particles are sintered at a temperature close to the melting point of the thermally conductive particles after the granulating process, the binder used in the granulating process disappears. Consequently, the cohesive force of the composite particles is not high after the sintering. In other words, the composite particles are fragile and cannot maintain their granulated state after the sintering, and are easily disintegrated.
If the thermally conductive particles are sintered at a temperature sufficiently higher than the melting point, the thermally conductive particles are fused and integrated with each other and composite particles having a high cohesive force can be obtained. However, as a result of the fusion and integration, they become massive and hard particles.
Patent Literature 3 discloses a powder composite that includes an inorganic powder such as alumina, a magnesium oxide, a boron nitride, or an aluminum nitride, and a thermosetting resin composite, and is processed into a powder, a granulated powder, or granules. However, since the particle size of the used inorganic powder is large and the thermosetting resin composite is used, the resin is hardened in the aggregates. Therefore, a hard powder composite having strong bonding is obtained.
Patent Literature 4 discloses a method for producing an aluminum nitride in which a composite particle powder that is obtained by coating the surface of an alumina particle powder with a surface modification agent and making a carbon powder adhere to that surface is heated and sintered at 1350 to 1750° C. under a nitrogen atmosphere (see [Patent Claims], and paragraphs [0034], [0042] and [0046]-[0049]).
Patent Literature 5 discloses a sintered spherical aluminum nitride powder having an average particle diameter of 10 to 500 μm and a porosity of 0.3% or higher. Specifically, Patent Literature 5 discloses a method for producing a sintered spherical aluminum nitride powder in which a slurry including an aluminum nitride powder including a powder having a primary particle diameter of 0.1 to 0.8 μm in a concentration of 10 mass % or greater based on the total mass, and a sintering assisting agent such as a lithium oxide or a calcium oxide is sprayed and dried, and then sintered at 1,400 to 1,800° C. (see Claims 1 and 4, and paragraph [0035]).
Similarly to Patent Literature 2, since the powder is sintered at a high temperature and the sintering assisting agent or the like is strongly bonded with the aluminum nitride in Patent Literatures 4 and 5, aggregates of hard aluminum nitride particles, or sintered and integrated hard and massive aluminum nitride particles are obtained.
Patent Literature 6 discloses secondary aggregates obtained by agglomerating primary particles of a scale-like boron nitride in an isotropic manner.
Specifically, Patent Literature 6 discloses a method for obtaining secondary aggregates having a porosity of 50% or smaller and an average pore diameter of 0.05 to 3 μm in which scale-like boron nitride is pre-sintered around 1800° C. and then granules consisting of primary particles that are obtained by pulverizing the pre-sintered boron nitride are sintered at 2000° C. (see paragraphs [0014], [0026] and [0027]).
Patent Literature 7 discloses spherical boron nitride aggregates obtained by agglomerating irregularly-shaped non-spherical boron nitride particles.
Patent Literature 8 discloses a silicon nitride-based sintered body.
Patent Literature 9 discloses a sintered spherical zinc oxide particle powder.
Patent Literature 10 discloses a thermally conductive composite material obtained by combining boron nitride particles and carbon fibers.
However, as the demands for heat radiation have grown, conventional thermally conductive particles or their granulated body cannot meet such demands any longer.
Therefore, it has been desired to develop a thermal conductivity-giving material capable of giving substantially the same level of a thermal conductivity as that of the conventional material by using a smaller amount thereof than that conventionally used or giving a higher thermal conductivity than that of the conventional material by using substantially the same amount thereof as that conventionally used.
Meanwhile, as for the thermally conductive member using thermally conductive particles, Patent Literatures 11 and 12, for example, disclose thermally conductive adhesive sheets using inorganic particles. To increase the thermal conductivity of such thermally conductive members, it is effective to raise the filling rate of the particles. However, the amount of the polymer material decreases as the amount of the particles increases. Therefore, the film-forming property and the base-material following property deteriorate. When a thermally conductive member is used as an adhesive sheet, in particular, the amount of the adhesive component decreases as the filling rate increases. Therefore, the adhesiveness deteriorates.
Patent Literatures 13 and 14 disclose methods for controlling the orientation of particles by applying a magnetic field or an electric field to the thermally conductive member in order to form contacts (thermal conduction paths) among particles while keeping the filling rate of the particles at a low level. However, these methods are not practical in consideration of the industrialization.
Patent Literature 15 discloses a method for developing a high thermal conductivity with a low filling amount by disposing secondary particles adjacent to each other in a coating and thereby forming tertiary aggregates. In this literature, a silane coupling agent is used as a binding agent for the granulation. Although the operability of the granulated body is improved by drying the secondary particles at 150° C. for four hours or longer and thereby carrying out a coupling reaction, the softness of the particles decreases. Consequently, both the thermal conductivity and the adhesive strength are unsatisfactory.
As described above, it is very difficult to achieve the high thermal conductivity, the excellent film-forming property, and the base-material following property of the obtained film in the conventional thermally conductive particles or the thermally conductive resin composites using their secondary particles (aggregates).
Further, as for those used for adhesive sheets, it is also very difficult to achieve the high thermal conductivity, the excellent film-forming property, and the base-material following property and the adhesiveness of the obtained film in the conventional thermally conductive particles or the thermally conductive resin composites using their secondary particles (aggregates).